Re: [PATCH 1/2] drm/display: hdmi: Only allow BPC values of 8, 10, 12 and 16

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Thursday, 18 June 2026 17:47:09 Central European Summer Time Maxime Ripard wrote:
> On Mon, Jun 08, 2026 at 01:19:07PM +0200, Nicolas Frattaroli wrote:
> > As per the comment in sink_supports_format_bpc(), CTA-861-F defines that
> > only bits-per-channel values of 8, 10, 12 and 16 are allowed for HDMI.
> > Allowing more than this has surprising consequences for the atomic check
> > phase. The HDMI state helpers may accidentally conclude that a sink
> > supports 11bpc if a caller asks for it.
> > 
> > Fix this by exiting early if the bpc value doesn't match one of those
> > given in the standard.
> > 
> > Fixes: 26ff1c38fc29 ("drm/connector: hdmi: Compute bpc and format automatically")
> > Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> > ---
> >  drivers/gpu/drm/display/drm_hdmi_state_helper.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> > index a331ebdd65af..8303475ec021 100644
> > --- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> > +++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> > @@ -420,6 +420,16 @@ sink_supports_format_bpc(const struct drm_connector *connector,
> >  		return false;
> >  	}
> >  
> > +	switch (bpc) {
> > +	case 8:
> > +	case 10:
> > +	case 12:
> > +	case 16:
> > +		break;
> > +	default:
> > +		return false;
> > +	}
> > +
> 
> That might be slightly personal, but I think if (bpc < 8 || bpc > 16 ||
> bpc % 2) is more readable

That would allow 14bpc, which wouldn't be correct.

Kind regards,
Nicolas Frattaroli

> 
> With that fixed
> Reviewed-by: Maxime Ripard <mripard@kernel.org>
> 
> Maxime
>